Author Topic: Thermistor behavior  (Read 756 times)

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Offline hmderek

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Thermistor behavior
« on: July 27, 2022, 08:26:11 AM »
So this is a relatively technical question.

I am currently playing around with adding NTC 10k thermistors to my weather station, alongside other types of thermometers I already use.
Right now I have a test setup with a Wemos D1 Mini (similar to the Arduino) and an Adafruit ads1115 as my analog to digital converter. I am using the sketch suggested by Adafruit: https://learn.adafruit.com/thermistor/using-a-thermistor

All this works quite well, and during daytime the thermistor temperature values line up nicely with the PT100 sensors in the same actively ventilated radiation shield.

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However, when the temperature drops, I see a deviation. See this chart incuding last night and this morning.

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The thermistor reads about 0,3-0,4 higher at around 9 degrees celsius. When temperatures rise, all sensors come into agreement again. I have not yet been able to see what happens when temperatures rise well above 20. Am waiting for this to happen.

In the mean time, my question: what could cause this? Specifically the thermistor reading 'too high' when temperatures decrease. Imprecision of the thermistor itself? Since the resistance increases when temperatures drop, I was thinking perhaps the inaccuracy of the thermistor becomes apparent? Looking for clues. Anyone have experience with this?

Will be adding additional thermistors with higher precision reference resistors soon. Will be interesting to see the results.
Davis VP2
Davis WeatherLink
Sensirion SHT35
PT100
NTC Thermistors
DS18B20
Apogee Instruments TS-100
Barani Meteoshield Pro
Davis 7714
MetSpec RAD14
Davis AirLink
Wemos D1 Mini micro controllers
https://blog.meteodrenthe.nl
https://meteodrenthe.nl
https://twitter.com/meteodrenthe

Offline davidefa

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Re: Thermistor behavior
« Reply #1 on: July 27, 2022, 10:30:54 AM »

Offline hmderek

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Re: Thermistor behavior
« Reply #2 on: August 08, 2022, 03:07:39 AM »
So since my original post Iíve been busy. And I think I also solved most of the issue. Wrote a post on it.

https://blog.meteodrenthe.nl/2022/08/07/getting-accurate-ntc-thermistor-readings-with-a-wemos-d1-mini/
Davis VP2
Davis WeatherLink
Sensirion SHT35
PT100
NTC Thermistors
DS18B20
Apogee Instruments TS-100
Barani Meteoshield Pro
Davis 7714
MetSpec RAD14
Davis AirLink
Wemos D1 Mini micro controllers
https://blog.meteodrenthe.nl
https://meteodrenthe.nl
https://twitter.com/meteodrenthe

Offline hmderek

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    • Meteodrenthe
Re: Thermistor behavior
« Reply #3 on: September 08, 2022, 04:13:55 AM »
Still at it, trying to optimize my thermistor readouts.

I fixed the issue with variable voltage input a while back. Now I've adjusted my code to use a different equation to determine temperature from the resistance of the thermistor.

For those interested, I documented a couple of things in this post: https://blog.meteodrenthe.nl/2022/09/07/the-b-parameter-vs-steinhart-hart-equation/
Davis VP2
Davis WeatherLink
Sensirion SHT35
PT100
NTC Thermistors
DS18B20
Apogee Instruments TS-100
Barani Meteoshield Pro
Davis 7714
MetSpec RAD14
Davis AirLink
Wemos D1 Mini micro controllers
https://blog.meteodrenthe.nl
https://meteodrenthe.nl
https://twitter.com/meteodrenthe

 

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